Mechanical and chemical-mechanical planarization processes remove material from the surface of semiconductor wafers, field emission displays and many other microelectronic substrates to form a flat surface at a desired elevation. FIG. 1 schematically illustrates a planarizing machine 10 with a platen or base 20, a carrier assembly 30, a polishing pad 40, and a planarizing solution 44 on the polishing pad 40. The planarizing machine 10 may also have an under-pad 25 attached to an upper surface 22 of the platen 20 for supporting the polishing pad 40. In many planarizing machines, a drive assembly 26 rotates (arrow A) and/or reciprocates (arrow B) the platen 20 to move the polishing pad 40 during planarization.
The carrier assembly 30 controls and protects a substrate 12 during planarization. The carrier assembly 30 generally has a substrate holder 32 with a pad 34 that holds the substrate 12 via suction, and an actuator assembly 36 typically rotates and/or translates the substrate holder 32 (arrows C and D, respectively). However, the substrate holder 32 may be a weighted, free-floating disk (not shown) that slides over the polishing pad 40.
The polishing pad 40 and the planarizing solution 44 may separately, or in combination, define a polishing environment that mechanically and/or chemically removes material from the surface of the substrate 12. The polishing pad 40 may be a conventional polishing pad made from a relatively compressible, porous continuous phase matrix material (e.g., polyurethane), or it may be an abrasive polishing pad with abrasive particles fixedly bonded to a suspension medium. In a typical application, the planarizing solution 44 may be a chemical-mechanical planarization slurry with abrasive particles and chemicals for use with a conventional non-abrasive polishing pad, or the planarizing solution 44 may be a liquid without abrasive particles for use with an abrasive polishing pad. To planarize the substrate 12 with the planarizing machine 10, the carrier assembly 30 presses the substrate 12 against a planarizing surface 42 of the polishing pad 40 in the presence of the planarizing solution 44. The platen 20 and/or the substrate holder 32 then move relative to one another to translate the substrate 12 across the planarizing surface 42. As a result, the abrasive particles and/or the chemicals in the polishing environment remove material from the surface of the substrate 12.
Planarizing processes must consistently and accurately produce a uniformly planar surface on the substrate to enable precise fabrication of circuits and photo-patterns on the substrate. As the density of integrated circuits increases, the uniformity and planarity of the substrate surface is becoming increasingly important because it is difficult to form sub-micron features or photo-patterns to within a tolerance of approximately 0.1 .mu.m when the substrate surface is not uniformly planar. Thus, planarizing processes must create a highly uniform, planar surface on the substrate.
In the competitive semiconductor and microelectronic device manufacturing industries, it is also desirable to maximize the yield of individual devices or dies on each substrate. Typical semiconductor manufacturing processes fabricate a plurality of dies (e.g., 50-250) on each substrate. To increase the number of dies that may be fabricated on each substrate, many manufacturers are increasing the size of the substrates to provide more surface area for fabricating additional dies. Thus, to enhance the yield of operable dies on each substrate, planarizing processes should form a planar surface across the substrate surface.
In conventional planarizing processes, however, the substrate surface may not be uniformly planar because the rate at which material is removed from the substrate surface (the "polishing rate") typically varies from one region on the substrate to another. The polishing rate is a function of several factors, and many of the factors may change throughout the planarizing process. For example, some of the factors that effect the polishing rate across the surface of the substrate are as follows: (1) the distribution of abrasive particles and chemicals between the substrate surface and the polishing pad; (2) the relative velocity between the polishing pad and the substrate surface; and (3) the pressure distribution across the substrate surface.
One particular problem with conventional planarizing devices and methods is that the deviation of the surface uniformity in a perimeter region of the substrate is generally much greater than that of a central region. In conventional planarizing processes, the polishing rate in a 5-15 mm perimeter region at the substrate edge is generally higher than the polishing rate in a central region. One reason for the difference in the polishing rate is that the relative velocity between the substrate and the polishing pad is generally higher in the perimeter region of the substrate than the central region. Another reason for the difference in the polishing rate is that the edge of the substrate wipes a significant amount of the planarizing solution off of the polishing pad before the planarizing solution can contact the central region. Conventional planarizing devices and methods, therefore, typically produce a non-uniform, center-to-edge planarizing profile across the substrate surface.
To reduce such center-to-edge planarizing profiles, several existing polishing pads have holes or grooves that transport a portion of the planarizing solution below the substrate surface during planarization. A Rodel IC-1000 polishing pad, for example, is a relatively soft, porous polyurethane pad with a number of large slurry wells approximately 0.05-0.10 inches in diameter that are spaced apart from one another across the planarizing surface by approximately 0.125-0.25 inches. During planarization, small volumes of slurry are expected to fill the large wells, and then hydrodynamic forces created by the motion of the substrate are expected to draw the slurry out of the wells in a manner that wets the substrate surface. However, even IC-1000 pads may produce significant center-to-edge planarizing profiles indicating that the perimeter of the substrate presses some of the slurry out of the wells ahead of the center of the substrate. U.S. Pat. No. 5,216,843 describes another polishing pad with a plurality of macro-grooves formed in concentric circles and a plurality of micro-grooves radially crossing the macro-grooves. Although grooved pads may improve the planarity of the substrate surface, substrates planarized with such pads still exhibit non-uniformities across the substrate surface indicating a non-uniform distribution of planarizing solution and abrasive particles under the substrate.
Other techniques for reducing the center-to-edge planarizing profile reduce the differences in the relative velocity between the perimeter and central regions. For example, one existing planarizing machine holds the polishing pad stationary and orbits the substrate in an eccentric pattern across the polishing pad. In another related planarization process, the substrate is held in a precession wafer holder that allows the substrate to precess with respect to the wafer holder during planarization. Although reducing the difference in the relative velocity across the substrate surface reduces the center-to-edge planarizing profile, existing planarizing machines may still produce significant deviations in the surface uniformity between the perimeter region and the central region.
In light of the results of conventional planarizing devices, the deviation of the surface uniformity in the perimeter region may be so great that it impairs or ruins dies formed in the perimeter region. Thus, because a defective 5-15 mm perimeter region affects a larger surface area and more dies on a 12-inch substrate than an 8-inch substrate, the center-to-edge planarizing profile significantly impacts the yield of larger substrates.